feat(operators): add RealBounds initializer and GaussianMutation

`RealBounds` (Initializer<Vec<f64>>) samples each variable uniformly in
its inclusive (lo, hi) range; panics if any bound has lo > hi
(spec §11.1).

`GaussianMutation` (Variation<Vec<f64>>) clones the first parent and
adds Normal(0, sigma) noise to every element; panics on sigma <= 0.0;
does not enforce bounds in v1 (spec §11.2).

src/prelude.rs

@@ -15,3 +15,5 @@ pub use crate::pareto::{
     Dominance, ParetoArchive, best_candidate, crowding_distance, non_dominated_sort,
     pareto_compare, pareto_front,
 };
+
+pub use crate::operators::{GaussianMutation, RealBounds};